1. Field of the Invention
This invention relates to the increased production of phosphoric acid, and, more particularly, to the increased production of phosphoric acid by the addition of gelatin to the reaction system.
2. Description of the Prior Art
Phosphoric acid is produced in the traditional wet process by the action of an inorganic acid on phosphate rock, producing phosphoric acid and the calcium salt of the acid used. Sulfuric acid is the preferred inorganic acid in that it produces an insoluble calcium sulfate which may be filtered. The calcium sulfate may be precipitated as a dihydrate (CaSO.sub.4.2H.sub.2 O) which is also known as gypsum, or as a hemihydrate (CaSO.sub.4.1/2 H.sub.2 O), or as an anhydrite (CaSO.sub.4).
The overall reaction for the dihydrate process is usually indicated as follows: EQU Ca.sub.5 (PO.sub.4).sub.3 F+5H.sub.2 SO.sub.4 +10H.sub.2 O.fwdarw.3H.sub.3 PO.sub.4 +5CaSO.sub.4.2H.sub.2 O+HF
There are also many secondary reactions which occur depending on the other minerals of the apatite series present in the phosphate rock. The dihydrate process involves the digestion of phosphate rock with strong sulfuric acid and weak recycled phosphoric acid for a given period of time, the time required to fully develop filterable and washable gypsum crystals. A single, or multiple, reaction tank may be utilized. The slurry is continuously drawn off, cooled, and filtered. The gypsum is finally washed with wash liquor containing approximately 2-5% P.sub.2 O.sub.5, and the washed gypsum is disposed of. This process produces approximately a 30% P.sub.2 O.sub.5 acid, which can then be concentrated to higher P.sub.2 O.sub.5 levels.
The principal objectives of any commercial phosphoric acid process should be:
1. To extract the maximum amount of P.sub.2 O.sub.5 from the phosphate rock. PA1 2. To precipitate a rapidly filtering and easily washable calcium sulfate. PA1 3. To produce a phosphoric acid having as high a P.sub.2 O.sub.5 content as possible. PA1 4. Continuous production over long periods at design capacities. PA1 5. Minimum operating, labor, and maintenance costs. PA1 6. Safe, clean plant operation, with absence of air and water pollution. PA1 1. sulfate ion content; PA1 2. phosphate ion content; and, PA1 3. type and amount of impurities present.
In the production of phosphoric acid by the wet process, the separation of calcium sulphate from phosphoric acid is an important step. This separation is generally accomplished by filtration using any number of well known devices. The large quantities of phosphoric acid produced in commercial plants requires filters of large area and complexity in order to insure good separation and recovery of the phosphoric acid. These filters are costly and difficult to maintain in good operating condition. In general, the concentration of phosphoric acid produced is limited to about 30% P.sub.2 O.sub.5 because higher concentrations cause lower filtration rates. These low rates are due to changes in the calcium sulfate crystals which are produced as by-product in wet process phosphoric acid. Phosphoric acid of greater than 45% P.sub.2 O.sub.5 is more desireable to produce, but the calcium sulphate hemihydrate crystals which are produced in this concentration range are very difficult to filter.
The size and shape of the calcium sulphate crystals are extremely important factors in proper filtration and washing. The three most important factors effecting crystal growth are:
Operating conditions and reactor design also influence crystal formation. The most desireable crystal forms would be rhombic or clusterlike, and the least desired would be needlelike or platelike crystals.
The effect of impurities on the filterability of calcium sulphate crystals has been discussed in Industrial and Engineering Chemistry Process Design and Development, 388, October 1966 (Gilbert, R.), and a summary of phosphoric acid filtration is presented in Chemical Engineering, 137, Aug. 15, 1966 (James, G. R.). An article by E. R. McCartney and A. E. Alexander entitled "The Effect of Additives upon the Process of Crystallization", Journal of Colloid Science 13, 383-396 (1958), describes the effects of gelatin on calcium sulphate crystal growth.
An object of this invention is the improvement of the filtration rate of the phosphoric acid-calcium sulphate slurry formed in the production of wet process phosphoric acid. This improvement will allow a smaller filtration area to be used in phosphoric acid plants and will also permit increased production in plants where current filtration area is limited. Since the filtration rate generally decreases as the concentration of the phosphoric acid increases, this invention will allow higher strength phosphoric acid to be produced without an increase in filtration area necessary. This invention also has application to the production of strong phosphoric acid such as that made in a hemihydrate process. The present invention improves the filtration rate of the slurry produced in the phosphoric acid wet process by from about 5% to about 30%, and this increased filtration rate is accomplished by adding relatively small amounts of gelatin to the reaction system. The addition of gelatin to the reaction system does not affect the color or viscosity of the phosphoric acid.